Motion compensated inter-frame coding has been widely adopted in various coding standards, such as MPEG-1/2/4 and H.261/H.263/H.264/AVC. While motion-compensated inter-frame coding can effectively reduce bitrate for compressed video, intra mode coding is still required to process an initial picture or to periodically insert I-pictures or I-blocks for random access or for alleviation of error propagation. Intra prediction exploits the spatial correlation within a picture or within a picture region. In practice, a picture or a picture region is divided into blocks and the intra prediction is performed on a block basis. Intra prediction for a current block can be relied upon pixels in neighboring blocks that have been processed. For example, if blocks in a picture or picture region are processed row by row from top to bottom and from left to right, neighboring blocks on the top and neighboring blocks on the left of the current block can be used to form intra predictor for pixels in the current block. While any pixels in the processed neighboring blocks can be used for intra predictor of pixels in the current block, very often only pixels of the neighboring blocks that are adjacent to the current block boundaries on the top and on the left are used. Furthermore, the intra predictor is usually designed to exploit spatial features in the picture such as smooth area (DC mode), vertical line or edge, horizontal line or edge and diagonal line or edge. Furthermore, a spatial feature often exists in both luma and chroma components. Therefore, the intra luma prediction mode can be used as a candidate for the intra chroma prediction mode. A respective intra predictor can be designed to exploit each feature accordingly.
As mentioned before, there are multiple intra prediction modes for intra mode coding. For each block, information associated with the selected intra prediction mode has to be conveyed to the decoder side so that a corresponding intra predictor can be applied to recover the intra-coded picture or picture region. When the block size is small, the side information associated with the intra prediction mode may be substantial. For chroma components of video data, the block size is usually smaller than the luma component. Therefore, the coding efficiency of intra prediction mode for the chroma components becomes more critical. Variable length codes have been used to code the intra chroma prediction mode in H.264/AVC standard and the High-Efficiency Video Coding (HEVC) standard being developed. Nevertheless, it is desirable to further improve the coding efficiency of the intra chroma prediction mode.